X-Git-Url: https://pd.if.org/git/?p=pdclib;a=blobdiff_plain;f=functions%2Fstdio%2F_PDCLIB_ftell64.c;fp=functions%2Fstdio%2F_PDCLIB_ftell64.c;h=650eda548d48051863868df176fffdb1584ecf51;hp=0000000000000000000000000000000000000000;hb=05556502a427911d4499c22b809ca7d366327d7c;hpb=852ad49d5888724e4e4f82a6d51d13b0e8029e28

diff --git a/functions/stdio/_PDCLIB_ftell64.c b/functions/stdio/_PDCLIB_ftell64.c
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+++ b/functions/stdio/_PDCLIB_ftell64.c
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+/* $Id$ */
+
+/* _PDCLIB_ftell64( FILE * )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <stdio.h>
+#include <stdint.h>
+#include <limits.h>
+
+#ifndef REGTEST
+
+uint_fast64_t _PDCLIB_ftell64( struct _PDCLIB_file_t * stream )
+{
+    /* ftell() must take into account:
+       - the actual *physical* offset of the file, i.e. the offset as recognized
+         by the operating system (and stored in stream->pos.offset); and
+       - any buffers held by PDCLib, which
+         - in case of unwritten buffers, count in *addition* to the offset; or
+         - in case of unprocessed pre-read buffers, count in *substraction* to
+           the offset. (Remember to count ungetidx into this number.)
+       Conveniently, the calculation ( ( bufend - bufidx ) + ungetidx ) results
+       in just the right number in both cases:
+         - in case of unwritten buffers, ( ( 0 - unwritten ) + 0 )
+           i.e. unwritten bytes as negative number
+         - in case of unprocessed pre-read, ( ( preread - processed ) + unget )
+           i.e. unprocessed bytes as positive number.
+       That is how the somewhat obscure return-value calculation works.
+    */
+
+    /* ungetc on a stream at offset==0 will cause an overflow to UINT64_MAX.
+     * C99/C11 says that the return value of ftell in this case is 
+     * "indeterminate"
+     */
+
+    return ( stream->pos.offset - ( ( (int)stream->bufend - (int)stream->bufidx ) + (int)stream->ungetidx ) );
+}
+
+#endif
+
+#ifdef TEST
+#include <_PDCLIB_test.h>
+
+#include <stdlib.h>
+
+int main( void )
+{
+    /* Tested by ftell */
+    return TEST_RESULTS;
+}
+
+#endif
+